Synapses are highly specialized for efficient synaptic transmission, with the transmitter release sites in the presynaptic nerve terminal being aligned with postsynaptic concentrations of the neurotransmitter receptor. At the developing neuromuscular junction, a nerve-derived factor called agrin directs the postsynaptic localization of the acetylcholine receptor, and our goal is to understand the molecular mechanisms that underlie agrin's action. Specifically, we will test the hypothesis that agrin regulates AChR localization by inducing phosphorylation of the AChR and altering its association with the intracellular scaffolding protein, rapsyn. We have the following specific aims: (1) To define the role of agrin-induced phosphoryation of the beta subunit in AChR localization. We will generate transgenic mice with a beta subunit lacking the phosphorylation site, and then determine whether this mutation impairs the synaptic localization of the AChR. We will also determine whether phosphorylation specifically regulates the anchoring, clustering, and/or stabilization of the receptor in the postsynaptic membrane. (2) To define the molecular mechanism of rapsyn-mediated AChR localization. Here, we will define rapsyn's binding site(s) on the AChR and determine whether binding is regulated by agrin. Moreover, we will investigate how agrin-induced changes in rapsyn/AChR association mediate receptor localization. This work is directly relevant to neuromuscular disorders such as congenital and autoimmune myasthenic syndromes, where there are severe deficiencies in AChR levels at the synapse, leading to impaired synaptic function and debilitating muscle weakness. Our work will help define the mechanisms involved in receptor localization and may suggest strategies to stabilize its localization in patients with myasthenic syndromes or other forms of neuromuscular disease.